The general purpose of a delay line filter is to perform filtering operations on temporally varying input signals. Most commonly, such temporally varying input signals are electrical in character, though they may originate in acoustical or other forms as well.
One form of the delay line filter is the transversal filter which comprises a delay line having taps spaced at suitable intervals of determinable time delays. Signals derived from the various taps are added and the resultant sum provides the output of the filter. Weighting factors, as may be represented by signal attenuation, for example, are sometimes applied to the individual taps.
Accordingly, if the input signal is electrical in character comprising a time varying voltage V.sub.s (t), the voltage output of the filter V.sub.o (t) may be expressed as ##EQU1## WHERE THE CONSTANTS A.sub.N AND .tau..sub.N REPRESENT THE RESPECTIVE WEIGHTING FACTORS, AND TIME DELAYS ASSOCIATED WITH THE N'TH TAP, N=1,2, . . . ,N, and N is the total number of taps. In present practice, electrical coaxial cables and surface acoustic wave devices are among the types of delay lines most commonly used in transversal filters.
However, coaxial cables suffer from the disadvantages of large signal distortion and attenuation at high frequencies, above approximately 100 MHz, and therefore are of limited usefulness for time delays much greater than one microsecond at such frequencies.
Surface acoustic wave devices also suffer from the disadvantage of severe attenuation at frequencies greater than a few hundred MHz and the fabrication of transversal filter devices which operate at bandwidths of several hundred MHz is accordingly difficult. Moreover, unwanted acoustic reflections can significantly degrade the performance of surface acoustic wave devices if the number of taps employed in a particular device is large.
Accordingly, there is a need for a delay line filter which will function to perform the desired operations without the disadvantages of present day and prior art delay line filters employing electrical coaxial delay lines and surface acoustic wave devices, for example. It is a contemplation of the present invention that fiber optic delay lines can be employed to obviate many of the disadvantages of prior art delay line filters.